Hi,

I want to draw an off axis parabolic mirror but i need a source (foused point) as input and collimated beam as output.

This means the optical axis and parabola axis are perpendicular to each other. I want to tilt the parabola and keep the source fixed.

Looking at knowlege base files for reference I can not understand how to decide the order decenter/tilt or tilt/decenter? Also please explain what is the meaning of before surface, after surface.